Add Kotlin code

Android Studio 3.0 and higher provides full support for
Kotlin, so it's easy to
add Kotlin files to your existing project and convert Java language code
to Kotlin. You can then use all of Android Studio's existing tools with your
Kotlin code, such as autocomplete, lint checker, refactoring, debugging, and
more.

Add Kotlin to an existing project

Click
File > New and select one of the various Android templates. If you don't
see the list of templates in this menu, first open the Project window and
select your app module.

In the wizard that appears, select Kotlin for the Source language.

Continue through the wizard, and you're done.

Alternatively, you can click File > New > Kotlin File/Class to create a
basic file. If you don't see this option, open the Project window and select
the java directory. The New Kotlin File/Class window provides several
choices for the file type, but it doesn't matter which one you choose because
Kotlin switches the file type automatically if you later change the declaration
type.

By default, new Kotlin files are saved into src/main/java/. You might find
it's easier to see both Kotlin and Java files in one location. But if you'd
prefer to separate your Kotlin files from your Java files, you can put
Kotlin files under src/main/kotlin/ instead. If you do, then you need to
include this directory in your sourceSets
configuration:

android {
sourceSets {
main.java.srcDirs += 'src/main/kotlin'
}
}

Convert existing Java code to Kotlin code

Open a Java file and select Code > Convert Java File to Kotlin File.

Or, create a new Kotlin file (File > New > Kotlin File/Class), and then
paste your Java code into that file—when prompted, click Yes to convert
the code to Kotlin. You can check Don't show this dialog next time, which
makes it easy to dump Java code snippets into your Kotlin files.